Conventional drum type washing machines comprise a drum which accommodates laundry and is provided in a water tub for storing washing liquid so that the drum is rotatable about a horizontal axis. The water tub is supported by a plurality of suspensions on a bottom plate of a casing in the drum type washing machine, whereby vibration is reduced during an operation. A suspension of the above-described type is known which uses as a working fluid a magneto-rheological fluid (hereinafter, “MR fluid”) which changes a viscosity according to the strength of a magnetic field.
One of conventional suspensions using the MR fluid includes a damper tube into which a piston rod is inserted and two pistons occupying two axial positions with respect to the piston rod respectively. An MR fluid accommodating chamber is defined between the pistons in the damper tube. The damper tube has a part that faces the MR fluid accommodating chamber and is provided with a magnetic generator. The MR fluid accommodating chamber is divided by the magnetic generator into two axial chambers. More specifically, the two divided chambers of the MR fluid accommodating chamber are provided at axial both sides of the magnetic generator respectively. The MR fluid fills both divided chambers.
The magnetic generator comprises an annular iron core on which a coil is wound so that magnetic field is caused. The magnetic generator and the piston rod form an MR fluid path therebetween. The drum type washing machine further comprises a damper tube side spring bracket mounted on a damper tube end where the piston rod protrudes. The piston rod has a protruding end on which a shaft side spring bracket is mounted. A coil spring is interposed between these spring brackets. The water tub is thus supported in the casing by the suspensions each of which uses the MR fluid.
When the water tub is vibrated vertically in the above-described water tub supporting construction, the damper tube is also reciprocated vertically or axially together with the water tub with expansion and contraction of the coil spring. In this case, the piston is reciprocated vertically relative to the MR fluid accommodating chamber in the damper tube. With the reciprocal movement of the piston, the MR fluid reciprocally flows through the MR fluid path between the divided axial chambers of the MR fluid accommodating chamber. The MR fluid then generates a damping force due to the viscosity thereof.
Upon energization of the coil of the magnetic generator, magnetic field is imparted to the MR fluid, thereby increasing the viscosity of the MR fluid. As a result, a frictional loss is increased when the MR fluid passes through the MR fluid path, whereupon the damping force is increased. In other words, the damping force can be adjusted by energizing the coil of the magnetic generator.